1. Field of the Invention
The present invention relates to a phase and base potential converter that converts a voltage of a first power supply base into a voltage of a second power supply base and a temperature-compensated crystal oscillator containing the converter as a phase and base potential converter of a frequency control signal, specifically to a construction of a temperature-compensated crystal oscillator having a frequency adjusting function by an external frequency control signal, incorporated in a communication device such as a portable telephone.
2. Description of the Related Art
Various electronic equipment now being used include the Vss base system in which the lower potential power supply (ground or VSS) is served as the base of the analog signal, and the Vdd base system in which the higher potential power supply (Vcc or Vdd) is served as the base of the analog signal. However, most electronic equipment adopts the Vss base system, and only some, such as a crystal watch, adopt the Vdd base system.
The reason why a crystal watch adopts the Vdd base system is as follows.
The crystal watch is an electronic device having a crystal oscillator integrated therein, and its power consumption has to be low. Accordingly, the crystal oscillator is constructed with complementary field effect transistor (CMOS).
Where the crystal oscillator comprises with CMOS transistor, in view of the oscillation starting characteristic and the power consumption, it is preferable to construct the oscillator so as to balance as much as possible the drive capability and gate capacitance, etc., between the p-channel MOS transistor and the n-channel MOS transistor constituting the oscillation inverter.
However, due to the difference between the mobility of electrons and that of holes, to obtain the equivalent drive capability necessarily requires the channel width of the p-channel MOS transistor be designed to be wider than that of the n-channel MOS transistor.
Accordingly, to balance the drive capability of the p-channel MOS transistor against that of the n-channel MOS transistor in the foregoing manner will lead to an imbalance of the gate capacity.
In order to control the imbalance to a low level as much as possible, it is necessary to reduce the difference between the mobility of electrons and that of holes as much as possible.
And, in the MOS transistor, as the impurity concentration in the channel region is high, the mobilities of electrons and holes become low.
Accordingly, it is preferable that the concentration of the p-type semiconductor substrate where the n-channel MOS transistor is formed be set higher than the concentration of the n-type semiconductor substrate where the p-channel MOS transistor is formed.
To achieve the foregoing, one of the simplest methods is to form the n-type semiconductor substrate as a starting material and on the surface thereof to selectively form a p-type diffusion region called the p-well as the region where the n-channel MOS transistor is formed.
In the case of a CMOS, all the pn junctions have to be reversely biased, and the semiconductor substrate is served as the base for the analog signal. Therefore, an electronic device constructed with a CMOS in which the n-type semiconductor substrate is formed as the starting material necessarily makes use of the Vdd base system in which the higher potential power supply is served as the base.
This is the reason that the crystal watch takes on the Vdd base system. In addition, not only the crystal watch, but crystal oscillators constructed with a CMOS, employ the Vdd base system in general.
Incidentally, in the temperature-compensated crystal oscillator incorporated in a communication device such as a portable telephone, an AT-cut crystal unit to cover the 10 MHz band is used as the source of vibration, a frequency adjusting circuit is applied to the source of vibration to make up a temperature compensation circuit, and the temperature characteristic of a tertiary curve of the AT-cut crystal unit is canceled. Thereby, the oscillation frequency of the temperature-compensated crystal oscillator is stabilized.
This type of temperature-compensated crystal oscillator has been comprised in most cases with an amplifier using bipolar transistors and a temperature compensation circuit composed of the series-parallel connection of a thermistor and capacitors; however recently, the temperature-compensated crystal oscillator constructed with a CMOS integrated circuit has become widely used.
The reason why the temperature-compensated crystal oscillator is constructed with a CMOS integrated circuit lies in that the improvement of performance and cost reduction can be achieved by freely using a technique such as analog signal synthesis.
In the case where the temperature-compensated crystal oscillator is constructed with a CMOS integrated circuit, from the consideration of the oscillation starting characteristic, for example, it is advantageous that the temperature-compensated crystal oscillator uses the Vdd base system, in which the n-type semiconductor substrate is formed as the starting material as mentioned above.
Incidentally, depending on the specification of the portable telephone, it is a usual exercise to apply an external frequency control signal to the temperature-compensated crystal oscillator and to thereby compensate for aging, etc.
In the temperature-compensated crystal oscillator incorporated in the portable telephone of such a specification, the temperature compensation and the external frequency control interfere each other, which can cause harmful effects such as lowering the frequency accuracy, etc.
One method for eliminating the mutual interference between the temperature compensation and the external frequency control is to synthesize the temperature compensation signal and the external frequency control signal and control a variable capacitance circuit, for example, by the composite signal thus synthesized, and thereby adjust the oscillation frequency of the crystal oscillating circuit.
However, the main body of a portable telephone uses the Vss base system, and the frequency control signal applied to the temperature-compensated crystal oscillator for compensating the aging, etc., is an analog voltage signal of the Vss (ground potential) base.
Therefore, in order to eliminate the mutual interference between the temperature compensation and the external frequency control by synthesizing the frequency control signal and the temperature compensation signal, the temperature-compensated crystal oscillator must be constructed with a CMOS integrated circuit of the Vss base.
However, to construct the temperature-compensated crystal oscillator with a CMOS integrated circuit of the Vss base will lead to increasing the imbalance between the gate capacitance of the p-channel MOS transistor and that of the n-channel MOS transistor constituting the oscillation inverter, as mentioned above, which invites disadvantages in terms of the oscillation characteristic such as the oscillation starting characteristic and power consumption, etc.
Therefore, if priority is given to the oscillation characteristic, the signal composition of the external frequency control signal and the temperature compensation signal was degraded.
That is, if the temperature-compensated crystal oscillator used in the environment of the Vss base system is constructed with a CMOS integrated circuit, there has been a problem in that either the oscillation characteristic or the signal composition for preventing the mutual interference had to be sacrificed.